$fields->addFieldToTab("Root.Content.Main", new DropdownField("ExistingTags", "Add existing tag/s",$this->TagsCollectionArray()), "Content");

function TagsCollectionArray() {

      // get the tags from all blog entries from the database
      $tags = DB::query("SELECT Tags FROM BlogEntry WHERE Tags != '';");

      // create a string with all tags (comma separated)   
      foreach($tags as $tag){
         $tagString .= $comma.$tag['Tags'];
         $comma = ',';
      }

      // creat an array from the comma separated string
      $allTags = explode(',',$tagString);
      
      // make sure there is no whitespace before or after the tag
      foreach($allTags as &$tag) {
         $tag = trim($tag);
      }

      // remove duplicates
      $allTags = array_unique($allTags);
      
      // get the tags from the selected blog entry
      $itemTags = split(" *, *", trim($this->Tags));

      // remove tags from the array that are already assigned to the blog entry
      $allTags = array_diff($allTags, $itemTags);
      
      // order the array alphabetically
      natcasesort($allTags);

      // add a 'select a tag' text as the first option
      array_unshift($allTags, 'select a tag');

      return $allTags;
   }

Requirements::javascript('blog/javascript/eventbehaviours.js');

// register an onchange behaviour
Behaviour.register({
   '#Form_EditForm_ExistingTags': {
      onchange: function() {
         Tags = document.getElementById("Form_EditForm_Tags");
         var comma = '';

         // the first option in the dropdown list is the text 'select a tag', so ignore this option
         if (this.selectedIndex > 0) {

            // add the selected tag, separated with a comma (if necessary)
            if (Tags.value != '') {
               comma = ', ';
            }
            Tags.value += comma + this.options[this.selectedIndex].innerHTML;
            
            // after the tag has been added to the blog entry remove it from the dropdown list
            this.options[this.selectedIndex] = null;
         }
      },
      initialize: function() {
         this.onchange();
      }
   }
});